[scottas]

I looked throught my code and saw, that anyone who understands html and javascript can simply add stuff to my website using same form information as mine. How to prevent it? By the way, I am using jQuery, so form submitted without reloading or redirecting page. What should I add to target page, to allow submitting only from my website?

[CTphpnwb]

You can't. This is one reason you can never trust the user. Always assume they're up to no good — always validate inputs and use prepared statements when dealing with databases.